Numerical assessment of flood hazard risk to people and vehicles in flash floods

Flash flooding often leads to extremely dangerous and sometimes catastrophic conditions 9 in rivers due to characteristics such as: short timescales, the limited opportunity for issuing 10 warnings, and the frequent high average mortality. Many past extreme flood events have been 11 accompanied by flash floods, and they have also been one of the main sources of serious loss of 12 human life among the world's worst natural disasters. Flash floods can also cause large loss of 13 property, such as the recent floods in Pakistan and the damage to vehicles in the 2004 Boscastle 14 flood in the UK. It is therefore desirable to be able to assess the degree of safety of people and 15 vehicles during flash floods using numerical models. In the current study, an algorithm for assessing 16 the flood hazard risk to people and vehicles has been integrated into an existing two-dimensional 17 hydrodynamic model capable of simulating flash floods. In the algorithm, empirical curves relating 18 water depths and corresponding critical velocities for children and adults, developed by previous 19 researchers, are used to assess the degree of people safety, and a new incipient velocity formula is 20 used to evaluate the degree of vehicle safety. The developed model was then applied to three real 21 case studies, including: the Glasgow and Boscastle floods in the UK, and the Malpasset dam-failure 22 flood in France. According to the analysis of model predictions, the following conclusions have 23 been obtained: (i) simulated results for the Glasgow flood showed that children would be in danger 24 of standing in the flooded streets in a small urban area; (ii) simulations for the Boscastle flood 25 indicated that vehicles in the car park would be flushed away by the flow with high velocity, which 26 indirectly testified the predictive accuracy of the incipient formula for vehicles; and (iii) simulations 27 for the Malpasset dam-failure flood showed that the adopted method for the assessment of people 28 safety was applicable, and some local people living below the dam would have been swept away, 29 2 which corresponded well with the report of casualties. Therefore, the developed integrated model 1 can be used to evaluate the flood hazard risk to people and vehicles in flash floods, and these 2 predictions can be used in flood risk management.